Electrochemical cells such as electrical storage cells or "batteries" may include electrochemically active materials in powder form. One known type of storage battery includes an anode of metallic lithium, a liquid electrolyte and a cathode incorporating a particulate electrochemically active material capable of taking up and releasing lithium ions. The particulate material typically is fixed to an electrically conductive current collector. During discharge, lithium ions from the anode pass through the liquid electrolyte to the particulate material of the cathode where the ions are taken up with release of electrical energy. An electrical current flows through the particulate material of the cathode, to the current collector and to the external load. During charging, the current flow and the flow of ions are reversed, so that current passes from the current collector to the particulate material and lithium ions pass from the particulate material through the electrolyte to the lithium anode. Operation of the cell requires interchange of ions between the liquid electrolyte and the individual particles of electrochemically active material, and also requires passage of an electrical current through the active material to and from the current collector.
The current collector may be a sheetlike metal element, whereas the particulate material may be distributed as a layer on the surface of this sheetlike collector. The particles typically are attached to the current collector and to one another by a polymeric binder. The binder maintains coherence of the particles and collector during cell assembly and during operation of the cell. The cell assembly operation typically involves forming the layered cathode current collector and particulate material into a sandwich like structure with other components such as a sheet-like porous separator and a sheet-like anode. This sandwich structure is then formed into a compact, convoluted structure as by folding or by winding into a spiral. The binder is particularly critical to successful completion of the convoluting process.
The binders utilized heretofore have been selected so as to resist chemical effects caused by the electrolyte. Thus, highly inert polymers such as polytetrafluoroethylene (PTFE) and polymers of ethylene propylene diene monomer (EPDM) have been used as binders. These polymers are electrical insulators. These polymers also retard movement of ions between the electrolyte and the particulate active material. As the proportion of binder in the layer of particulate material is increased, the mechanical stability of the layer is also increased. However, this increase in mechanical stability is also accompanied by a decrease in the electrical conductivity and a decrease in ionic access to and from the powdered material. Both of these factors tend to decrease cell performance. Accordingly, the amount of binder utilized heretofore has been limited and selected to provide a compromise between these competing considerations.
Electrodes of this type are usually solvent cast by dispersing the binder and the particulate, active material in a solvent, depositing this dispersion onto the current collector and evaporating the solvent. This procedure usually creates a "skin" or region of particularly high binder concentration at the surface of the particle layer remote from the current collector. This "skin" further aggravates the problem of ionic access.
Additionally, many particulate electrochemically active materials will spontaneously react with the ordinary ambient atmosphere. For example, lithiated molybdenum disulfide is pyrophoric; it will ignite spontaneously when exposed to ordinary ambient air. Other electrochemically active particulate materials will not react so violently, but will nonetheless be damaged by exposure to air. Electrodes incorporating such materials heretofore have generally been handled under inert atmospheres. Use of such an inert atmosphere is costly.
Accordingly, there have been significant needs heretofore for improvements in electrochemical cells and improvements in methods of making such cells.